FIG. 10 is a sectional view showing the structure of a conventional solar cell device 102.
The solar cell device 102 has a structure in which a transparent oxide electrode 12 is formed on the surface of an insulating substrate 10 which is a transparent glass substrate, and a p-type semiconductor layer 18, a buffer layer 20, an intrinsic semiconductor layer 22, an n-type semiconductor layer 24, and a metal electrode 26 are laminated in that order on to the surface of the transparent oxide electrode 12 into a laminated structure.
The insulating substrate 10 transmits light incident from the surface on the side thereof (the lower side in the drawing), on which the transparent oxide electrode 12 is not formed, to the transparent oxide electrode 12.
The transparent oxide electrode 12 is formed to lead light (mainly sunlight) incident through the insulating substrate 10 to the intrinsic semiconductor layer 22 through the p-type semiconductor layer 18 and the buffer layer 20 and to keep ohmic contact with the p-type semiconductor layer 18.
The p-type semiconductor layer 18 is a layer composed of a p-type semiconductor, which is provided to lead carriers, produced in the intrinsic semiconductor layer 22 by the incident light, to the transparent oxide electrode 12. The buffer layer 20 functions as a buffer layer for preventing a forbidden band width of the intrinsic semiconductor layer 22 from narrowing due to the mixing of p-type impurities (boron) contained in the p-type semiconductor layer 18, into the intrinsic semiconductor layer 22. The intrinsic semiconductor layer 22 is a layer made of an intrinsic semiconductor for producing carriers by absorbing incident light. The n-type semiconductor layer 24 is a layer made of an n-type semiconductor provided to lead the carriers produced in the intrinsic semiconductor layer 22 to the metal electrode 26. The metal electrode 26 is connected with an interconnection for taking out electromotive force.
Next, the fabricating method of the aforesaid conventional solar cell device will be described using FIG. 11 through FIG. 15.
First, tin oxide film is formed on the insulating substrate 10 to form a transparent oxide electrode 12, and thereafter a photoresist 13 is applied on the entire surface of the tin oxide film. The photoresist 13 is exposed and developed with a predetermined mask to remain in a region which is to be the solar cell device 102.
Next, as shown in FIG. 11, the transparent oxide electrode 12 is etched by means of a reactive ion etching system with the above photoresist 13 as an etching mask and with hydrogen iodide (HI) and argon (Ar) used as the raw material gas. Removing the photoresist 13 makes a state where the transparent oxide electrode 12 is provided on the surface of the insulating substrate 10 as shown in FIG. 12.
Subsequently, the p-type semiconductor layer 18 is formed on the entire surface of the insulating substrate 10 so as to cover the transparent oxide electrode 12, as shown in FIG. 13, by the plasma CVD (chemical-vapor deposition) method. At this time, mono-silane (SiH.sub.4) and diborane (B.sub.2 H.sub.6) are used as the raw material gas. Methane gas (CH.sub.4) is simultaneously introduced to form silicon carbide in the p-type semiconductor layer 18, thereby preventing the forbidden band width of the p-type semiconductor layer 18 from narrowing and the light conversion efficiency from lowering. Sequentially, the buffer layer 20 is formed over the entire surface of the p-type semiconductor layer 18. This is carried out by the plasma CVD method with mono-silane (SiH.sub.4) and methane gas (CH.sub.4). The intrinsic semiconductor layer 22 is next formed on the entire surface of the buffer layer 20. This is also carried out by the plasma CVD method with monosilane (SiH.sub.4) as the raw material gas.
Moreover, as shown in FIG. 14, the n-type semiconductor layer 24 is formed on the entire surface of the intrinsic semiconductor layer 22. This is performed by the plasma CVD method with mono-silane (SiH.sub.4) and phosphine (PH.sub.3) as the raw material gas. Thereafter, a metal film 25 which becomes the metal electrode 26 is formed on the entire surface of the n-type semiconductor layer 24 by the sputtering method and a photoresist 15 is applied on the entire surface of the metal film 25.
The photoresist 15 is exposed and developed with a predetermined mask, as shown in FIG. 15, to remain only in a region which is to be the solar cell device 102. Then, the metal film 25 and the respective layers laminated thereunder are etched and removed by the reactive ion etching method using the photoresist 15 as an etching mask, and the photoresist 15 used for the etching mask is also removed.
Consequently, the solar cell device 102 can be fabricated as shown in FIG. 10, in which all layers from the p-type semiconductor layer 18, the buffer layer 20, the intrinsic semiconductor layer 22, the n-type semiconductor layer 24 to the metal electrode 26 are laminated in that order on the transparent oxide electrode 12.
Incidentally, although the solar cell device 102 in the prior art can be fabricated by the above fabricating method, the solar cell device 102 has a structure using tin oxide as the transparent oxide electrode 12, and thus it has the following disadvantages regarding the structure. More specifically, tin oxide is a chemically very active substance and has a characteristic of being easy to react with a semiconductor layer laminated on the transparent oxide electrode 12. Accordingly, interdiffusion of atoms tends to occur between the transparent oxide electrode 12 and the p-type semiconductor layer 18, and thus there arise disadvantages that transparency of the transparent oxide electrode 12 deteriorates, resulting in a decrease in transmittance and that deterioration in film quality of an amorphous semiconductor layer (especially, the intrinsic semiconductor layer 22) causes a decrease in open circuit voltage.
Generally, it is required to improve efficiency of converting light to electrical energy as much as possible in a solar cell. Since the conversion efficiency of energy is obtained by the ratio of energy which incident light possesses to the maximum output which is obtained by the product of open circuit voltage and short circuit current, a decrease in open circuit voltage causes a decrease in the maximum output of the solar cell, resulting in a decrease in conversion efficiency of energy. Accordingly, it is an extremely important subject to stabilize the surface of the transparent oxide electrode for the prevention of a decrease in open circuit voltage due to the deterioration of the film quality thereof to thereby improve the output characteristics of the solar cell.
An object of the present invention is to improve output characteristics of a solar cell device including enhancement of open circuit voltage by solving the disadvantages as described above in the solar cell device and the fabricating method of the same.